In order to prepare colour-altering cosmetic substances, particularly for keratin fibres such as hair, a person skilled in the art is familiar with various dyeing systems depending on the requirements the colouring must satisfy. For permanent, intense dyeing with corresponding fastness properties, substances called oxidation dyes are used. Such dyes typically contain oxidation dye precursors, also called “developer and coupler components”, which combine with each other to form the actual dyes under the effects of oxidising agents such as hydrogen peroxide. Oxidation dyes are notable for their excellent, long-lasting colouring results. Besides dyeing, many consumers are particularly fond of lightening their hair or giving it a more blond appearance. For this, the natural or synthetic dyes that colour the hair fibres are mostly bleached oxidatively by the action of corresponding oxidising agents, such as hydrogen peroxide.
In order to develop a satisfactory dyeing and lightening effect, oxidative dyeing and lightening agents generally need an alkaline pH value during application. Optimal results are obtained particularly with pH values between 8.5 and 10.5.
Until the present time, the alkalising agent of choice for adjusting these pH values has been ammonia. With ammonia, not only is it possible to set the pH range required for forming the dye, but ammonia lends body to the hair more effectively than any other known alkalising agent. At the same time, ammonia functions as a penetrating agent and a penetrating auxiliary agent—also more effectively than all other commercially available alkalising agents.
This is why, when ammonia is used in oxidative dyes, significantly better results are obtained in terms of colour intensity and grey coverage than with other alkalising agents (such as potassium or sodium hydroxide, alkanolamines, or carbonates such as sodium carbonate or potassium carbonate).
As the colours are more intense from the outset, so the fastness properties of the hair dyes produced with the aid of ammonia are also improved. In particular, coloured hair exhibits the best washing fastness properties when ammonia has been chosen as the alkalising agent.
The use of ammonia brings so many different technical application advantages that ammonia is used in a large number of commercially available oxidative dyeing and lightening products even despite its unpleasant, acrid smell.
Extensive efforts to temper the smell of ammonia are already known from the literature. In this context, approaches for minimising the odour fall into three basic groups: according to the literature, the first option is to change the alkalising substance, and thus partially or completely replace ammonia with odourless agents.
Accordingly, many formulations exist that, for example, use a mixture of ammonia and monoethanolamine or monoethanolamine alone as the alkalising agent. However, the result of completely or partially replacing ammonia is that the dyes generally do not penetrate the hair as well, which in turn leads in particular to less satisfactory grey coverage and poorer washing fastness, as described in the preceding. If the development of particularly long-lasting shade variations is important, the use of monoethanolamine is therefore not a viable option.
Documents WO 2006060570 and WO 2006060565 suggest the use of carbonates or carbonate sources as alkalising agents for preparing oxidative dyes with low odour. However, it is also known from the literature that when used together with oxidising agents, carbonates can severely damage hair. The additional hair damage caused by the carbonates may not be a major cause for concern when the dye is used on untreated or undamaged hair, but for individuals who regularly colour or lighten their hair, severe cumulative damage may be caused. If more pronounced lightening and/or regular dyeing is desired, the use of carbonates also does not represent an acceptable alternative.
A second general approach for reducing the smell of ammonia consists in the addition of special perfumes that are intended to hide the smell. This route is adopted in WO 2005/110499 for example. However, perfumes can be unstable under the alkaline storage conditions, and there is a risk that the fragrances may be decomposed or the structure thereof may be altered during storage, resulting in an unpredictable alteration of the smell. Since such alterations often do not become noticeable until several months or even years later, the use of new and/or unknown perfumes is to be regarded as unreliable.
A third general approach for diminishing the smell of ammonia consists in an optimisation of the formulation. In this case, the objective is to select the carrier components of the formulation in such manner that they ensure as much of the ammonia as possible is retained inside the formulation, thereby minimising its odour. However, it is also known that the formulation, the lipids, emulsifiers, tensides it contains, and the viscosity thereof have a significant effect on colouring performance. If the formulation is modified, it is therefore essential to ensure that the colouring performance is not impaired thereby.
As examples of such, JP 2007191459 suggests the use of cationic tensides, phosphate esters and aliphatic alcohols in order to reduce the smell of ammonia in hair dyeing agents. JP 2003040750 discloses that the ammonia odour in lightening agents is particularly weak when at least 5% of a crystalline component is added to them.
It is particularly difficult to devise a solution that will minimise the odour for the entire period for which the application lasts. The period for which the user of hair colouring products is in contact with the dyeing agent lasts from the production of the application mixture, throughout the time when it is applied to the hair, and the time required for it to take effect until the formulation is washed out. Given typical working times from about 30 to about 45 minutes, the entire process can take up to 90 minutes, in extreme cases even as long as two hours. Providing a solution for covering the smell of ammonia that remains effective for this entire period poses an extremely difficult challenge. Precisely in this field, there is significant need for improvement, and as yet there is no optimal solution known from the prior art for reducing the smell of ammonia.
The problem addressed by the present invention was therefore to provide oxidative agents for dyeing and/or lightening the colour of hair that have a less pronounced smell of ammonia. At the same time, the use of such agents should not lead to any loss of colouring performance, particularly grey coverage, or washing fastness. Moreover, the use of such agents should not be associated with greater hair damage.